1. Field of the Invention
The present invention relates to a capacitor power supply unit with a plurality of capacitors serially connected to each other, and more particularly to charge control of capacitor power supply. The capacitor power supply unit can be used for a heating device, a copier, a digital multifunction peripheral, a printer, a facsimile device, and the like.
2. Description of the Related Art
Copier, printers, or the like form images on a recording medium such as a plain paper and a medium for an overhead projector (OHP). An electrophotographic method is employed for the image formation in terms of high-speed image formation, image quality, and cost. The electrophotographic method is a method of forming a toner image on a recording medium, and fixing the toner image on the recording medium with heat and pressure. As a fixing method, a heat roll method is most broadly adopted at present in terms of safety. The heat roll method is a method of bringing a heating roller into contact with a pressing roller with pressure to form a mutual contact portion called a nip portion, and heating the recording medium with the toner image thereon at this nip portion. More specifically, the heating roller is heated by a heating element such as a halogen heater, and the pressing roller is arranged so as to face the heating roller. Recently, environmental problems become significant, which results in progressing of energy saving also in image forming apparatuses such as copiers and printers. A point which is not negligible to think about the energy saving of the image forming apparatus is power saving of a fixing device for fixing toner on the recording medium. An image forming apparatus for implementing energy saving is proposed in, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-184554. The energy saving in the image forming apparatus is implemented by using a high-capacitance capacitor such as an electric double layer capacitor as an auxiliary power supply for the fixing device to increase the maximum supply power.
A rated voltage of the electric double layer capacitor is dependent on a decomposition voltage of an electrolytic solution and is very low such as about 2.5 volts. Therefore, when the electric double layer capacitor is used as an electrical storage device, it is general to serially connect a plurality of the capacitors to increase the rated voltage and use the capacitors as a module. However, when a group (a module or a unit) of the capacitors serially connected to each other is to be charged or discharged, fluctuation in shared voltages occurs caused by fluctuation of capacitance and leakage current among the capacitors, and when each capacitor voltage exceeds the rated voltage, the excess causes degradation of the capacitance of the capacitor.
Therefore, when the module with the capacitors serially connected to each other is to be charged or discharged allowing for the fluctuation of capacitance and leakage current among the capacitors, the module has to be used by setting the capacitor voltage to a charging voltage lower than the rated voltage so that the capacitor voltage does not exceed the rated voltage. However, energy that can be stored in the electric double layer capacitor is based on a relation of W=CV2/2. Therefore, if a terminal voltage is made lower than the rated voltage, decrease of charging energy is proportional to the square of the voltage. For example, if 2.0 volts is full charge with respect to the rated voltage of 2.5 volts, only 64% of chargeable energy can be charged.
To resolve this problem, JP-A Nos. H06-261452, 2004-336919, 2005-86900, 2002-142369 disclose parallel monitor circuits each of which detects each capacitor terminal voltage of the electric double layer capacitors serially connected, determines that the terminal voltage becomes a predetermined value, and restricts charging of the capacitor. A capacitor charging circuit is described in the cited document 4. The capacitor charging circuit is such that when the parallel monitor circuit bypasses the capacitor to restrict the charging, the parallel monitor circuit causes a photocoupler to emit light to generate a signal indicating completion of charging, and finishes the charging by the charging circuit when all the photocouplers generate a charging completion signal.
However, in these charging methods, if there is large fluctuation of capacitance among individual capacitors serially connected, it takes a comparatively long time from when an initial capacitor reaches the rated voltage and the parallel monitor is thereby activated until all the capacitors are charged to the rated voltage. Moreover, the power consumed in the parallel monitor is a large amount of heat generation expressed by terminal voltage×bypass current. Therefore, it is necessary to provide a circuit element or a radiating mechanism appropriate for such a large heat generation. Described in the cited document 5is that, for equalizing charging voltages of capacitors, a unit with capacitors serially connected to each other is charged for a predetermined time, and after that, a voltage of a reverse polarity is applied to the unit for a fixed period to perform full discharge. However, it is necessary to provide a reverse-polarity charging circuit or a charging-polarity switching circuit for complete discharge of the whole capacitors in a short time, which causes hardware and charge/discharge control to become complicated. Furthermore, a large amount of power is consumed for charging or discharging for resetting used to completely discharge the whole capacitors.